1. Field of the Invention
The present invention relates to a laser machining method and a laser machining apparatus for machining holes on a workpiece by irradiating pulsed laser beams to the workpiece.
2. Related Art
Conventionally, a laser machining method is arranged so as to concurrently machine two holes by a first optical mirror system composed of a pair of rotary mirrors and a second optical mirror system composed of a rotary mirror and a rotary polarizing beam mixer by concurrently inputting an S-polarized second laser beam and a P-polarized first laser beam to one fθ lens as disclosed in Japanese Patent Laid-Open No. 2004-249364 for example.
In case of CO2 laser for example, a material that is liable to absorb the CO2 laser may be efficiently machined because it is possible to increase energy density of the laser beam. However, it is difficult to machine a copper plate because its surface reflects most of the CO2 laser when the CO2 laser is irradiated to the copper plate. Then, in the case of machining a via hole for an electrically connecting surface and inner copper conductive layers (referred to as ‘copper layers’ hereinafter) formed respectively on the surface and inside of a printed circuit board in which the copper layers and a resin layer (an insulating layer made of resin or of resin and glass fibers, referred to as ‘insulating layer’ hereinafter) are laminated, a hole is made on the surface copper layer in advance and a laser beam is irradiated to the hole to remove any insulating substance and to make the via hole reach to the inner copper layer. Therefore, this method has required many machining steps.
Meanwhile, because UV laser light is liable to be absorbed by many materials such as metals, organic and inorganic materials, it can machine a composite material relatively easily as compared to the CO2 laser. Therefore, it is unnecessary to make a hole through the surface copper layer in advance and thus a number of machining steps may be reduced in machining a via hole for electrically connecting the surface and inner copper layers of a printed circuit board. Then, the machining method using the UV laser is becoming the main stream process in machining printed circuit boards.
In the case of the UV laser, its energy per one pulse is small even though oscillating frequency of a laser oscillator is high. However, it can efficiently machine a copper layer for example even though the energy per one pulse is small by contracting an outer diameter of the laser beam and by increasing energy density per unit area. However, there is a case in which the bottom of the copper layer melts and a through hole is made if the energy density is too high.
For example, a hole of 40 μm in diameter for connecting copper layers on the surface and inside of a printed circuit board in which the copper layers and an insulating layer are laminated can be made by burst machining (a machining method of repeatedly irradiating a laser beam to the same position, and which is referred to as punching here because it is similar to punching) by using the UV laser of the same diameter, i.e., 40 μm. It is noted that the energy density in machining the copper layer is 8 J/cm2, the energy density in machining the insulating layer is 1 J/cm2 and pulse frequency is 30 KHz.
(1) In the case of machining the surface copper layer when a thickness thereof is 5 μm or less and a thickness of the insulating layer is 25 μm or more:
Delamination occurs at the boundary of the surface copper layer and the insulating layer and the copper layer tends to delaminate if the laser beam is irradiated successively for more than a certain number of times. That is, it is necessary to limit the number of times of irradiation to a number set in advance in order not to cause any delamination at the boundary of the copper layer and the insulating layer.
(2) In the case of machining the surface copper layer when the thickness of the surface copper layer is 5 μm or more and the thickness of the insulating layer is 25 μm or more:
Irradiating the laser beam successively for more than a certain number of times damages the insulating layer under the copper layer, thus enlarging an overhang of the surface copper layer and worsening adhesion of plating. That is, it is necessary to limit the number of times of irradiation to a number set in advance in order to reduce the overhang of the copper layer.
(3) In the case of machining the insulating layer when the insulating layer is made of only resin:
Irradiating the laser beam successively for more than a certain number of times makes a hole in the shape of a beer barrel. That is, it is necessary to limit the number of times of irradiation to a number set in advance in order to form a hole whose sidewall is straight.
(4) In the case of machining the insulating layer when the insulating layer contains glass fibers:
Irradiating the laser beam successively for more than a certain number of times makes a hole in the shape of a beer barrel in which the glass fibers protrude into the hole. That is, it is necessary to limit the number of times of irradiation to a number set in advance in order to form a hole whose sidewall is smooth and is straight.
Still more, the copper layer tends to delaminate from the insulating layer when an irradiation pitch (intervals of irradiation position of the laser beam, which is also called a pulse pitch) is reduced also in trepanning (a method of machining a hole whose diameter is larger than that of the laser beam by irradiating the laser beam by a plurality of times on a circumferential orbit) under the same conditions with those described above.
Therefore, it becomes necessary to prevent the copper layer from delaminating from the insulating layer without lowering the machining efficiency and while preventing concentration of heat in any cases as a laser machining method.
Then, in case of (1) described above for example, machining quality has been maintained by repeating cyclic machining, i.e., irradiating the laser beam by a number of pulses set in advance to each machining hole in a set of a plurality of holes, by a necessary number of times.
A concrete machining time may be set in the case of the cyclic machining as follows.
It is necessary to irradiate the laser beam by 40 pulses in machining the insulating layer of 40 μm thick and made of only resin with a pulse frequency of 30 kHz and energy density of 1 J/cm2. Here, a time necessary for positioning the optical mirror is presumed to be 2 kHz. It is also presumed to machine a plurality of holes concurrently while irradiating 10 pulses each to one hole. In this case, it takes 3.2 ms to machine one hole because the number of times of positioning of the optical mirror is four, and four times machining is carried out per each machining hole. Still more, it takes 5.1 ms to machine one hole in the case of irradiating five pulses each to one hole because 8 times of positioning and 8 times of machining are carried out.
Still more, the larger the diameter of the laser beam (beam diameter) and the higher the machining speed, the more the amount of decomposed scattering substance increases in unit time. Then, when the laser beam passes through the decomposed scattering substance at high temperature, index of refraction of the laser beam may vary. Then, due to that, there is a case in which a beam mode changes and precision of shape of the machined hole deteriorates. That is, although it is possible to make a hole whose wall face is steep, i.e., a hole whose bottom diameter is close to an entrance diameter, when the beam mode is a Gaussian mode (laser beam whose energy intensity is in a shape of Gaussian line), there is a case in which the bottom diameter of the hole is much smaller than the entrance diameter when the beam mode changes. Accordingly, the laser machining method is also required to suppress the generation of the decomposed scattering substance without lowering the machining efficiency.
Accordingly, it is an object of the invention to provide a laser machining method and a laser machining apparatus for machining workpieces while keeping excellent machining accuracy and quality without lowering their machining efficiency.